What, since that one time they descended on planet Earth to destroy the entire human race you mean? Hmm... Good question. On paper, the Reapers are a race of dragon-like creatures who serve to 'correct' temporal paradoxes by removing the offending element before generally devouring everything within it. Their first and only appearance to date was in Series 1's Father's Day when they were drawn to Earth in 1987 after Rose Tyler saved her father from his untimely demise. In a nutshell, they basically hate it whenever time is changed which is all well and good but isn't that what the Doctor is always doing? Well, yes and no. The argument of 'fixed points' comes into it and it didn't help matters that there were actually two versions of the Doctor and Rose present at the scene of what should have been Peter Tyler's death. Rose's emotions got the better of her in the end when she was seen by her previous self and everyone knows that's one of the biggest school boy errors that any time traveller can make. Suffice to say, the Doctor wasn't best pleased but everything turned out for the best in the end, despite the fact that Pete ended up dying again so nothing really changed anyway. Rose learned a lot in the process, though, and that's what really counts, right? So yeah, the Reapers are always mad but they were particularly peeved in this instance. That's pretty much all there is to say. Next?